In order to increase the power and data rates available from laser diodes, multi-element, individually addressable arrays of laser diodes are sometimes used. These can be of the diffraction-limited single-mode type or arrays of broad area emitters. The broad area emitters are diffraction-limited in one dimension, referred to as the "narrow" dimension and act as a wide area source in the other, or "wide" dimension. The advantage of the broad area emitters, also referred to as "stripe" laser diodes, is the much higher output power possible. Because of this high power, these devices are manufactured with their positive side bonded to the heat sink, since the thermal conductivity of the substrate material is much lower than that of the heat sink. Since the connections to the individual diodes also have to come out from the positive side, there is a conflict, and a compromise, between the heat sinking needs and the interconnections.
A further problem with multi-element, individually addressable diodes is reliability. Since each diode is focused onto a separate modulator cell and then onto a light sensitive material or sensor, a failure or defect in any one of the many elements renders the device useless. Prior art attempts to overcome these problems by coupling a wide area laser diode to a multi-channel modulator were only partially successful since they required a high degree of collimation in both axes. The wide area laser diodes can only be collimated in one axis. As an example of prior art, U.S. Pat. No. 4,577,932 uses a wide area laser diode and an acousto-optic modulator. However, it is limited to operation in a pulsed mode and to relatively narrow diodes.
It is an object of this invention to utilize very wide area laser diodes (typically 10 mm wide) in conjunction with electro-optic multi-channel modulators to form the equivalent of a multi-element individually addressable laser diode but without the limitation of such a device. Such wide area diodes are constructed from a plurality of small elements in parallel, i.e., the elements are not individually addressable.
Another object is to have a device with higher reliability than a multi-element individually addressable diode by eliminating the trade-off between interconnection and heat sinking and by eliminating the effects of data dependent thermal cycling. Reliability is further enhanced by being tolerant to local defects and failures since the near field radiation from the diode is not being imaged in the long dimensions of the diode.
Still another object is to provide multi-channel laser diode systems which are less susceptible to back reflection (from the material being exposed) than laser diodes having narrow emitters.